Roofing materials usually contain a surface coating of base material, covered by coarse protective particulate matter such as gravel. In time, a thick coating of dust (often 1/2 inch or more in thickness) builds up and must be removed when the roof is cleaned.
Repair of roofs often involves resaturation or recoating of the base material, and the residual dust reduces the penetrating qualities of the resaturant or coating before reaching the roof mat for which it is intended. Since hot applications cure rapidly on contact with cool roof surfaces, the hot material cures on top of the dust if the roof mat is not properly cleaned. The dust also absorbs the resaturant material, reducing or inhibiting the ability of the resaturant to saturate the roof mat. Clearly careful and thorough cleaning is essential.
One traditional way of cleaning a roof is to hand brush the edges of confined areas, power sweep the roof, remove and dispose of the roof debris, power sweep again to remove the fine dust materials (fines), remove and dispose of the fines, and finally blow the roof area to remove residual fines. Sometimes a third power sweep is required. This technique often produces clouds of blowing residue, which pollutes the atmosphere and usually resettles on the roof.
A more recent approach to cleaning roofs is to rough clean or remove the loose gravel and debris from the roof area to be upgraded or repaired, and then power sweep to loosen the fine dust that has been packed on the roof mat over the years. Finally the roof is vacuumed to pick up the loosened fine dust. The vacuum system safely confines and contains fine dust and other pollutants before they are released into the atmosphere.
However it has been found that when the roof is wet, the fines are solid and require extensive agitation to loosen, but still cannot be picked up by the vacuum unit before resettling.